The present invention relates to a method of casting powder so designed that metal powder or ceramic powder is dispersed into a dispersing medium to form a slurry and the slurry is cast.
A casting method has been known in the art as a method of molding metal powders such as 2% Ni-98% Fe powder, SUS 316 powder and stellite powder, ceramics such as alumina, silicon carbide, silicon nitride, zirconium, molybdenum carbide and titanium carbide, and mixed powders of ceramics and metals such as tungsten carbide-cobalt and titanium carbide-nickel. This method is one in which a starting powder is dispersed into a dispersing medium to form a slurry and the slurry is poured into a mold having liquid absorbing properties, thus causing the mold to absorb a part of the dispersing medium and form a cast thickness and thereby producing a molding.
In this case, the starting powder used is ground and mixed with a dispersing medium such as water and a small amount of a dispersing agent in a ball mill to form slurry, and in this case the slurry was stabilized by stirring the mixture for several days, adjusting the water content and viscosity and then subjecting to vacuum defoaming.
On the other hand, a gypsum mold subjected to preliminary mold processings such as the application of a mold release agent is assembled and the slurry prepared in the above-mentioned manner is poured into the mold, thereby forming a cast thickness along the mold area due to the absorption of the water by the mold and growing the cast thickness with time.
With the slurry poured into the mold, the mold continues to absorb the water and the water content of the cast thickness is decreased gradually. Thus, the casting is increased in hardness and it is also shrunk, thereby permitting its release from the mold.
At this stage, the casting is removed from the mold so that if necessary, the casting is dried after it has been processed in the raw state. On the other hand, the gypsum mold which has absorbed the water is dried for repeated use.
As the dispersing medium used in preparing the slurry, it is known to use various alcohols in place of water.
Also, it is known to use a permeable mold made from a mold material other than gypsum, e.g., a plastic material which is excellent in mechanical strength and wear resistance, a metal-ceramic fiber composite material or ceramic material such as silicon carbide.
In addition, a mold of compressed powder material is formed by inserting a mold core, ramming powder material and then removing the mold core and the removal of a casting from the mold is effected by knocking down the mold to restore the compressed powder material to the original powder form. The restored powder material is dried and used repeatedly.
This type of casting method for powdered metals and ceramics has the following disadvantages.
(1) The operations including from the casting to the removal from the mold requires a long period of time. While the required time is dependent on the shape, size, etc. of a desired molding and thus we cannot absolutely say so, eight hours is required for casting a water-type slurry of alumina into a cylindrical shape of 12 mm.phi..times.20 mm, for example.
(2) The drying operation of a molding requires a long period of time. For instance, twenty-four hours are required for air drying the above-mentioned molding.
(3) A mold drying operation is necessary. In addition, this operation requires a long period of time. Therefore, this operation should preferably be eliminated.
(4) The drying operation tends to cause a distortion or cracks in the molding. The reason is that the dispersion liquid is vaporized from the surface of the molding and lost, thus causing voids inside the molding so that a capillary force acts at the vapor-liquid surfaces and the vapor-solid-liquid points therein and the adjacent particles attract one another. This phenomenon causes the shrinkage of the molding to proceed from the surface layer portion and tends to cause a distortion or cracks in the molding.
To overcome the above disadvantages (2) and (4), the applicant has applied for patents on powder molding methods via Japanese Patent Applications No. 60-261274) and No. 60-129960 (b) (Laid-Open Specification No. 61-287702). The powder molding method of Patent Application No. 60-100433 is a powder molding method in which the dispersing medium used in the molding of a molded shape from metal or ceramic powder by casting is extracted and removed in a supercritical carbon dioxide atmosphere and the major constituent of the dispersing medium is extracted and removed by a supercritical carbon dioxide. The powder molding method of Patent Application No. 60-129960 (Laid-Open Specification No. 61-287702) is one which is characterized in that the dispersing medium used in the molding of a molded shape from metal or ceramic powder by casting is extracted and removed by a liquid carbon dioxide of a temperature which is not less than -30.degree. C. and not more than 31.1.degree. C., that is, the dispersing medium is extracted and removed by a liquid carbon dioxide of a temperature immediately below the critical temperature or a temperature range between -30.degree. C. and 31.1.degree. C. and the carbon dioxide is removed by the succeeding pressure reduction. In accordance with this method the dispersing medium removing time is greatly reduced to 1.5-2.0 hours in the case of a molding of 12 mm.phi..times.20 mm, for example.
Further, in the method of the former (a) the super-critical carbon dioxide has no surface tension so that during the reduced-pressure vaporization operation following the replacement of the dispersing medium, no capillary force acts in the molding thus causing no distortion or cracks in the molding. Also, in accordance with he latter method, during the reduced-pressure vaporization operation there are the occurrence of vapor-liquid interfaces and the action of capillary force and therefore there is the possibility of causing a distortion or cracks in the molding unless a considerable amount of time is spent.
It will thus be seen that while the method disclosed in the former (a) overcomes the disadvantages stated in the above (2) and (4), the disadvantages of the above (1) and (3) still remain unsolved. On the other hand, the method disclosed in the latter (b) has a problem with respect to the operation of removing the liquid carbon dioxide from the molding so that while the disadvantage of the above (1) and (2) still remain unsolved and the disadvantage of the above (4) is not overcome completely.
As described hereinabove, the conventional cast molding methods of powder materials have various disadvantages.